Doctor or smoothing blade for treating coated sheets



March 1936- R. 1.. JENKINS El AL DOQTOR 0R SMOOTHING BLADE FOR TREATING COATED SHEETS Filed Feb. 18, 1953 FII INVENTOR 5 ATTORNEY leg - [11L Jenkins Patented Mar. 10, 1933 DOCTOR R SMOOTHING BLADE FOR TREATING COATED SHEETS Russell L. Jenkins, Anniston, and Thomas S.

Foley, Birmingham, Ala., assignors to Swami Research Incorporated, a corporation of Alabama Application February 18, 1933, Serial ,No. 657,326 1 Claim. (01. 91-53) The object of this invention is to provide a new and improved doctor or smoothing blade, particularly adapted for use in connection with sheet or paper coating or impregnating machines.

A further object is tdprovide a heated doctor which may be used for smoothing paper or other sheet materiaLwhich has been previously coated with resins, waxes or mixtures of the same, and

ywhich will cause a smooth, glossy surface to be formed thereon, thereby giving maximum transparency to the coated paper.

The coating of paper, either of the glassine or tissue type, with resins, waxes or mixtures thereof, or of other transparent sheets therewith, is

usually carried out by passing a continuous web or sheet of the paper through a saturating bath of the coating composition, then through squeeze rolls which may remove some material from the sheet, and then over doctors or, scrapers which remove a further excess of the composition from the sheet.

For most paper co'ating purposes a molten mixture of resins and waxes is employed for saturating the sheet; however, in certain cases it may be desirable to lower the temperature of application of such composition by the addition of a solvent thereto. Such solvent may be removed from the surface of the coated paper by passing the sheet through a heated chamber or by blow-.

ing air over same. The use of our improved heated doctor herein disclosed will, in such cases, assist in the evaporation of any residual solvent and leave the surface of the paper in a particularly smooth condition.

We have found that the ordinary flat surfaced or cylindrical doctors when used with resinous coating compositions, especially those containing waxes, leave the surface covered with minute projections or irregularities. Such-surface irregularities tend to cause-dispersion of light falling on the surface and thereby decrease the transparency of the sheet. We have now found that if a. doctor blade be equipp d with a curved or cylindrical surface terminating in a straight sharp edge, the coated paper, after having been drawn under tension over such surface and edge, possesses a remarkably smooth surface-the sheet having a high degree of transparency.

' The construction a and use of our improved doctor will be apparent by reference to the accompanying drawing in which Figure 1 is a plan view; c Figure 2 is a section along the linen-11 of [Figure 55 Figure 3 is another sectional view showing how the coated sheet is passed over the doctor when m use. i

Figure 4 is a sectional view taken alongthe line IV--IV of Figure 3. v Referring now to the drawing, numeral l0 designates a supporting bar which, if desired, may also serve as a source of heat for the doctor blade. For this purpose, if steam is available, the supporting bar It] is a hollow tube with appropriate connections II and l2 td serve as inlet 10 and outlet, respectively, for the steam, ,To bar I 0 the curved arcuate doctor blade l3 isflaflixed by any suitable means, such as welding or soldering. The essentials of doctor blade l3 are the curved surface l4, preferably highly polished, 15 and the cut-back face [5 forming, at the juncture with the curved surface, a sharp edge I6. For most satisfactory results the angle ll .between face l5 and a tangent to curve Id at the edge l6 should be somewhat less than 90". We 20 have found that if angle I1 is made approximately 60, satisfactory results may be obtained.

Referring to Fig. 3 of the drawing, we'show a preferred arrangement of a pair of our improved doctors for smoothing a coating. on both sides of a 25 sheet of paper so as to develop smoothness and a maximum transparency of surface. A sheet" of paper [8, which has previously been passed through a coating bath (not shown) is passed through a pair of squeeze rolls 20 and is then 30 passed over one of our improved doctors Illa disposed to' smooth the lower surface of the paper. From thence the paper passes under a second doctor [0b, disposed to smooth the upper surface of the paper. The two doctors, Illa and lllb, are 35 so disposed with' relation-to each other, and the curved blade 13 of the doctor Illa is so disposed, that the sheet of paper I8 passes first upwardly over the curved surface [4 and is then'drawn downwardly, as seen'in the drawing, at an angle 40 to bring it sharply across the sharp edge It of the blade. I

For most of the hard varieties of coating compositions, the doctor blade'l3 is so disposedthat a considerable portion of thecurved surface I4 is in contact with the sheet lB'as it passes thereover, and a maximum degree of smoothing of the coating is had. This is ordinarily referred toin the art as the wrap. The amount of wrap, however, is variable, which is. inter-related with other factors to be explained more in detail hereafter. I x

From the doctor llla, the sheet l8 passes downwardly' under the doctor 10b, and the blade l3 of the latter doctor is also so disposed that the upper surface of the sheet receives a treatment similar to that already described for the lower surface. The blade I3 is also so disposedthat it serves to draw the sheet l8 downwardly to bring the lower surface sharply over the sharp edge [6 of the doctor Illa. After passing the doctor Mb, another roll 2| is employed to direct the sheet l8 upwardly and thus bring the upper surface of the sheet l8 sharply over the edge I6 of the doctor lob, thus providing a treatment similar to that given the lower surface, as heretofore described.

The doctors Illa and "lb are adjustable angularly and vertically with respect to each other in any suitable manner, an example of which is shown in Fig. 4 of the drawing. Each,of the doctors is mounted at the ends in a cup bracket 22 secured to the frame 23 of the machine by a threaded shank 24 passing through a slot 25 in the frame of the machine and provided with a nut 26. The doctor is an ularly adjustable in the cup bracket and fixed in its adjusted position by means of a set screw 21. It will be apparent that the angle of approach of the paper to the curved surface as well as its leaving angle with respect to the edge of the blade may be varied within any desired limits by the adjustments described.

Having described the general features of construction of our improved doctor blade, it re mains to describe the several features peculiar to its operation, since its adjustment is somewhat critical.

The factors which we have found to be of importance in the operation of our improved apparatus are as follows:-

(a) Tension of the sheet passing over the blades.

(b) Temperature of the blades.

(0) Amount of wrap. v

((1) Speed of the sheet over the blades.

(e) Properties of the hot coating composition, such as viscosity or softening point.

Referring to (a) above, in operating a coating machine employing our improved doctor blades, it is first desirable to adjust the tension of the sheet to the point where the blade tends to smooth out lines or other irregularities on the coated sheet but which tension is still insufiicient to cut off wax from the sheet. It should be borne in mind in this connection that when the coated or saturated sheet arrives at the point in the.

machine where our doctor blades are to function, it is presumed that the amount of coating material remaining on the sheet is the amount finally desired on such sheet, any excess thereof having been removed by squeeze rolls or scraping blades of the ordinary type. If the tension on the moving sheet is too great, wax will accumulate on the blade at the point where sheet l8 comes into contact with surface I4 of the blade. 'By descreasing the tension on the sheet, this effect can be prevented. However, if the tension is insumcient, the maximum degree of smoothing will not be attained.

Referring to (b), the "temperature at which the blades are operated is of importance. Arelatively high temperature of the blades will increase the tendency of the blades to cut wax, while a lower temperature will decrease such tendency. The optimum temperature for any given coating composition is that at which effective smoothing of the coatirg is obtained without removing any substantial amounts of the composition from the sheet.

Referring to (c), the amount of wrap of the sheet on the blades determines the velocity at which the sheet may be drawn over the blades and also the self-cleaning tendency. Since the function of the blade is to smooth a thermoplastic material by means of heat and pressure, sufflcient wrap is desired so that a softening of the coating on the sheet is obtained in the time interval during which such sheet is in contact with the blade surface l4.

By self-cleaning tendency is meant the ability of the blade to pass particles of dirt or foreign matter which inevitably lodge in the coating bath or on the paper, and which, unless removed, cause lines or streaks on the sheet which seriously detract from the appearance of the finished sheet. By a proper choice of wrapand tension the blade can be caused to freely pass such foreign particles; in other words, the blade becomes self cleaning.

Referring to (d), the speed of the coated sheet over the blades is, as has already been .said, a factor dependent upon the amount of wrap, the properties of the coating composition and the temperature of the blades. The rate of travel of the sheet should be sumcient to allow the softening of the coating to take place, so that the smoothing of the surface is completebefore the sheet leaves the trailing edge l6 of the blade.

Referring to (e), as has already been mentioned, the coating composition to be applied to paper or other sheet material usually consists of a thermoplastic mixture of resins, waxes and plasticizers, the viscosity of which varies with the properties of the material entering into the composition. In general it may be said that the and passes over the blade surface to the trailing edge IS, an increasingly greater pressure is exerted by the blade on the sheet. Such increasing pressure permits the surface of the thermoplastic coating composition to gradually attain the temperature of the blade, causing a gradual'softening and flowing of thecoating, due to the temperature and pressure to which it is subjected. At the edge is just before the sheet leaves the blade, the unit pressure of the blade,on the coating is a maximum. After the sheet has passed over edge Hi, all pressure on the sheethas been released. The important point in regard to. the successful operation of our improved blade on coated sheets, particularly those coated with resins and waxes, is the combined effect of the sliding motion of such coated sheets over the curved surface,the gradually increasing pressure exerted thereby, which pressure rises to a maximum at the trailing edge 16, after which all pressure is suddenly released. In order that the pressure be a. maximum at the edge I6, it is necessary that the take off angle of the sheet at the edge l6 be of substantial size.

By take off angle is meant the angle between a tangent to surface I4 at the edge It and sheet l8 when the intersection between the direction line of the'paper and the line of the tangent is at the edge Hi. If the take off angle is small, i. e., zero or approximately zero, the beneficial ple matter of varying the inclination of the blade until maximum smoothing is obtained.

In this connection it should be noted that if the imaginary tangent line to the curved surface l4 at edge I6 and the direction line of the sheet l8 do not intersect at edge l6, then the take off from the blade will be tangential to curve I d, at a point other than l6. When operating in this manner, the blade causes ripple-marks on the coated surface, which decrease the transparency of the sheet. The regular orientation and structure of these ripple-marks also causes a noticeable astigmatism of objects viewed through the sheet. Such defects in the coated sheet are readily overcome by the use of our improved doctor blade in the manner set forth.

While we have described our invention in but one form, it will be obvious to those skilled in the art that it is not so limited, but is susceptible of various changes and modifications without departing from the spirit thereof; and we desire, therefore, that only such limitations shall be placed thereupon as are imposed by the prior art, or as are specifically set forth inthe appended claim.

What is claimed is,

A doctor for smoothing coating material on a tensioned moving web comprising a heated tubular support and a parallel edged smoothing blade tangentially attached thereto, said smoothing blade having a curved section of uniform thickness terminating in asharp straight edge, said curved section being adapted for tangential contact with said web at a point spaced from said edge, a transverse section taken at said edge exhibiting an angle of substantially 60, said blade being adapted to maintain continuous contact with said tensioned moving web from the point of tangential contact of said web on said curved section to the point of edge contact.

RUSSELL L. JENKINS. THOMAS S. FOLEY. 

